Voltage transients can induce very high currents and voltages that can penetrate electrical devices and damage them, either causing hardware damage, such as semiconductor burnout, or electronic upset, such as transmission loss or loss of stored data. The voltage transients produce large voltage spikes with high peak currents (i.e, overvoltage). The three basic overvoltage threats are electrostatic discharge, line transients, and lightning. Electrostatic discharge typically occurs when static charge dissipates off the body of a person in direct physical contact with an operating electronic system or an individual component, such as an integrated circuit chip. Line transients are surges in AC power lines. Line transients can also occur due to closing a switch or starting a motor. Lightning strikes can strike stationary objects, such as a building, or mobile objects such as aircraft or missiles. Such strikes can suddenly overload a system's electronics. At peak power, each of these threats is capable of destroying the sensitive structure of an integrated circuit chip.
Various overvoltage protection materials have been used previously. These materials are also known as nonlinear resistance materials and are herein referred to as voltage variable materials. In operation, the voltage variable material initially has high electrical resistance. When the circuit experiences an overvoltage spike, the voltage variable material quickly changes to a low electrical resistance state in order to short the overvoltage to a ground. After the overvoltage has passed, the material immediately reverts back to a high electrical resistance state. The key operational parameters of the voltage variable material are the response time, the clamp voltage, the voltage peak and peak power. The time it takes for the voltage variable material to switch from insulating to conducting is the response time. The voltage at which the voltage variable material limits the voltage surge is called the clamp voltage. In other words, after the material switches to conducting, the material ensures that the integrated circuit chip, for example, will not be subjected to a voltage greater than the clamp voltage. The voltage at which the voltage variable material will switch (under surge conditions) from insulating to conducting is the switch voltage. These materials typically comprise finely divided conductive or semiconductive particles dispersed in an organic resin or other insulating medium. For example, U.S. Pat. No. 3,685,026 (Wakabayashi, et al.), U.S. Pat. No. 4,977,357 (Shrier) and U.S. Pat. No. 4,726,991 (Hyatt et al.) disclose such materials.
Voltage variable materials and components containing voltage variable materials have been incorporated into overvoltage protection devices in a number of ways. For example, U.S. Pat. Nos. 5,142,263 and 5,189,387 (both issued to Childers et al.) disclose a surface mount device which includes a pair of conductive sheets and voltage variable material disposed between the pair of conductive sheets. U.S. Pat. No. 4,928,199 (Diaz et al.) discloses an integrated circuit chip package which comprises a lead frame, an integrated circuit chip protected by an electrode cover which is connected to ground on one side, and a variable voltage switching device including the voltage variable material connected to the electrode cover on the other side. U.S. Pat. No. 5,246,388 (Collins et al.) is directed to a device having a first set of electrical contacts that interconnect with signal contacts of an electrical connector, a second set of contacts that connect to a ground, and a rigid plastic housing holding the first and second set of contacts so that there is a precise spacing gap to be filled with the overvoltage material. U.S. Pat. No. 5,248,517 (Shrier et al.) discloses painting or printing the voltage variable material onto a substrate so that conformal coating with voltage variable material of large areas and intricate surfaces can be achieved. By directly printing the voltage variable material onto a substrate, the voltage variable material functions as a discreet device or as part of associated circuitry.
The above U.S. Patents referred to are incorporated herein by reference.
Although the prior art discloses various materials and devices, there is a continuing and long felt need to provide improved cost-effective voltage variable materials and devices of more consistent performance properties to prevent variations in the clamp voltage under various conditions in which the materials and devices are used.